April 17, 2014

Geologists who analyzed 40 meteorites that fell to Earth from Mars unlocked secrets of the Martian atmosphere hidden in the chemical signatures of these ancient rocks. The results will help guide researchers’ next steps in understanding whether life exists, or has ever existed, on Mars and how water—now absent from the Martian surface—flowed there in the past. Their study shows that the atmospheres of Mars and Earth diverged in important ways very early in the 4.6 billion year evolution of our solar system.

The researchers measured the sulfur composition of 40 Mars meteorites—a much larger number than in previous analyses. Of more than 60,000 meteorites found on Earth, only 69 are believed to be pieces of rocks blasted off the Martian surface.

The meteorites are igneous rocks that formed on Mars, were ejected into space when an asteroid or comet slammed into the red planet, and landed on Earth. The oldest meteorite in the study is about 4.1 billion years old, formed when our solar system was in its infancy. The youngest are between 200 million and 500 million years old.

Studying Martian meteorites of different ages can help scientists investigate the chemical composition of the Martian atmosphere throughout history, and learn whether the planet has ever been hospitable to life. Mars and Earth share the basic elements for life, but conditions on Mars are much less favorable, marked by an arid surface, cold temperatures, radioactive cosmic rays, and ultraviolet radiation from the Sun.

Still, some Martian geological features were evidently formed by water – a sign of milder conditions in the past. Scientists are not sure what conditions made it possible for liquid water to exist on the surface, but greenhouse gases released by volcanoes likely played a role.

Sulfur, which is plentiful on Mars, may have been among the greenhouse gases that warmed the surface, and could have provided a food source for microbes. Because meteorites are a rich source of information about Martian sulfur, the researchers analyzed sulfur atoms that were incorporated into the rocks.

In the Martian meteorites, some sulfur came from molten rock, or magma, which came to the surface during volcanic eruptions. Volcanoes also vented sulfur dioxide into the atmosphere, where it interacted with light, reacted with other molecules, and settled on the surface.

Sulfur has four naturally occurring stable isotopes, or different forms of the element, each with its own atomic signature. Sulfur is also chemically versatile, interacting with many other elements, and each type of interaction distributes sulfur isotopes in a different way. Researchers measuring the ratios of sulfur isotopes in a rock sample can learn whether the sulfur was magma from deep below the surface, atmospheric sulfur dioxide or a related compound, or a product of biological activity.

Using state-of-the-art techniques to track the sulfur isotopes in samples from the Martian meteorites, the researchers were able to identify some sulfur as a product of photochemical processes in the Martian atmosphere. The sulfur was deposited on the surface and later incorporated into erupting magma that formed igneous rocks.

The isotopic fingerprints found in the meteorite samples are different than those that would have been produced by sulfur-based life forms.The researchers found the chemical reactions involving sulfur in the Martian atmosphere were different than those that took place early in Earth’s geological history. This suggests the two planets’ early atmospheres were very different, Franz said.

The exact nature of the differences is unclear, but other evidence suggests that soon after our solar system formed, much of Mars’ atmosphere was lost, leaving it thinner than Earth’s, with lower concentrations of carbon dioxide and other gases. That is one reason why Mars is too cold for liquid water today—but that may not always have been the case, said Franz.

“Climate models show that a moderate abundance of sulfur dioxide in the atmosphere after volcanic episodes, which have occurred throughout Mars’ history, could have produced a warming effect which may have allowed liquid water to exist at the surface for extended periods,” Franz said. “Our measurements of sulfur in Martian meteorites narrow the range of possible atmospheric compositions, since the pattern of isotopes that we observe points to a distinctive type of photochemical activity on Mars, different from that on early Earth.”

Periods of higher levels of sulfur dioxide may help explain the red planet’s dry lakebeds, river channels and other evidence of a watery past. Warm conditions may even have persisted long enough for microbial life to develop.

The team’s work has yielded the most comprehensive record of the distribution of sulfur isotopes on Mars. In effect, they have compiled a database of atomic fingerprints that provide a standard of comparison for sulfur-containing samples collected by NASA’s Curiosity rover and future Mars missions. This information will make it much easier for researchers to zero in on any signs of biologically produced sulfur, Farquhar said.

The Daily Galaxy via Nature.com and University of Maryland College of Computer, Mathematical, and Natural Sciences

I can't read the copyright on the image, but it doesn't look like it says NASA. And even if it is a NASA image, that doesn't answer my question. I've seen many, many photos from various Mars (and other planetary) missions, and the reason I asked my question was because it is *not* obvious that this is a photo; it looks much more like an illustration, and NASA does issue images that fall into each of the categories in my question. If you don't know the answer, just say so, but getting snippy does not win you any fans.

As far as I known other than the Earth, moon and Mars, we do not know the chemical compositions of any of the other planets, moons, or asteroids. How do we really know that these meteorites really come from Mars?

Quite contrary, we we know quite well the composition od other bodies like Mercury,Venus , various asteroids and moons of outer planets.
Scientists can even tell that certain rock came from Vesta and not other asteroid
So if rock looks like Martian,it most likely came from Mars or some other place that is like Mars ,that we are not aware of.Second option not likely

@Pm
Vesta is one of the largest asteroids in the belt between Mars and Jupitar. The Dawn spacecraft did a fly by in 2012 but as far as I know did not land there and take any rock samples. There is a higher probability that meteors classified as coming from Vesta actually come fom Ceres as it is the largest asteroid. Much less is known about Ceres because it has only been viewed from a distance so far. Until a chemical composition of the entire solar system is done it will remain only a educated guess where meteorites come from. It was proved only just a few years ago that water once exsisted on Mars so how can we say with 100 percent certainty where meteors come from?

You need not land on a body to know its chemical composition (at least surface's composition )
All you need is a Sun's light reflected of a surface of such planet, moon or asteroid. You can see a chemical composition by looking what is "missing" from original sunlight. These are called absorption lines in light spectrum. It's well known what absorption lines belong to which element .
It is called spectroscopy. My description is extremely simplistic but I hope you get the idea.

And believe it or not , all nearby bodies in Solar system have quite distinctive chemical compositions. It all comes down to a place where certain body acquired its material , at what time etc.

And while we are at it you can't possibly have two bodies more different that Ceres and Vesta. Ceres is a kind of ice world , while Vesta is a piece of heavy rock, not unlike Moon in composition. :-)

Other interesting possibility:
If you know that a rock you hold , came from meteor that is part of well known meteor stream, or for example you have a film that shows where it entered atmosphere , at what angle etc. you may get to know this meteorite's exact orbit before it crashed.

Then you may literally unwind its orbit to trace it millions of years back, and discover by a way of computer program that it was chopped off Vesta 500 millions years ago because their simulated orbits meet in this point of simulation